Reddit, the popular web forum site, appears to have received a national security letter in the last year — at least, that’s the strong implication of the site’s deletion of a so-called warrant canary.

Warrant canaries — named after the coal-mining tradition of using canaries as an early warning system for carbon monoxide leaks — are an ingenious, if legally dubious, way of getting round the secrecy mandate that comes with some surveillance requests from the U.S. government.

When the FBI makes extraordinary search requests for a customer’s records, the firm on the receiving end generally cannot legally disclose the fact — the authorities want their surveillance to stay secret.

So some companies have taken to regularly publishing a piece of text stating they have never received such a request. When they fail to publish that piece of text, the implication is that someone came knocking with a national security letter.

In Reddit’s case, the company publishes an annual transparency report about government requests for user data. The 2015 report, published Thursday, lacked the usual warrant canary.

Reddit CEO Steve Huffman, who frequents the site under the username “spez,” wrote in the thread discussing the report that he had been “advised not to say anything one way or the other,” as to whether the canary’s absence meant there had been a request.

This is true. As Moxie Marlinspike, co-creator of the Signal encrypted messaging app, wrote a couple years back, many lawyers advise that warrant canaries are not an effective way to get around the feds’ demand for secrecy.

“Every lawyer I’ve spoken to has indicated that having a ‘canary’ you remove or choose not to update would likely have the same legal consequences as simply posting something that explicitly says you’ve received something,” he wrote at the time.

However, civil rights groups such as the Electronic Frontier Foundation do still promote the warrant canary idea as being legal. Sites such as Tumblr and Medium use warrant canaries, and you can see a regularly updated list of those canaries on a service called Canary Watch.

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